SUCTION OF LIQUIDS. 165 trunk, when not in use, is coiled up in a spiral beneath the head; as is shown in fig. 87, representing the head of a Butterfly, a, of which the eye is seen at c, the base of the antennæ at b, the palpi at e, and the trunk at d. In some of the Fly tribe, the trunk attains a length several times greater than that of the body, as shown in fig. 88, representing a dipterous (two-winged) insect from the Cape of Good Hope, which sucks the juices of a single kind of flower, the length of whose tube just equals that of its long proboscis. 166 DEVELOPMENT OF TEETH. Mastication. 174. The act of Mastication, or the mechanical division of the alimentary matter, is effected in most of the higher animals, by the Teeth; which are implanted in the jaws, and are so fixed as to act against one another, with a cutting, crushing, or grinding power, according b a Fig. 89.-DEVELOPMENT OF TEETH. с to the nature of the food on which they have to operate. The manner in which they are formed is worthy of note. In Man, who may be taken as a fair example, each tooth is developed in the interior of a little mem a, the gum; b, the lower jaw; c, angle of the branous sac, which is lodged in the thickness of the jaw d jaw; d, dental capsules. d bone; as seen in the accompanying figure, which represents half the lower jaw of a very young infant, from which the outside has been removed. This sac, which is named the dental capsule (a, fig. 90), is composed of two membranes, abundantly furnished with blood-vessels; and it encloses in its interior a little bud-like protuberance, b, in which ramify a great number of nervous filaments and minute vessels, c. The matter composing this little body, which is termed the pulp, is gradually converted into the dentine (§ 54) of the tooth, which in Man constitutes nearly its whole structure; this conversion takes place first at its highest points, d, d. The crown or upper portion of the tooth receives a covering of enamel (§ 54). Gradually the process of conversion extends more and more to the interior of the pulp; and at last the whole is changed into dentine, with the exception of a small portion that still remains, occupying what is termed the cavity of the tooth, which is frequently laid open by decay of its external wall. The fang of the tooth, which is the part last formed, receives an envelope of cementum (§ 54), which invests it up to the part at which the enamel begins. As the a b Fig. 90.-DENTAL DEVELOPMENT OF TEETH. 167 root of the tooth is developed, the crown is gradually pushed upwards, so as to press against the upper portion of the capsule and the gum by which this is covered. These parts yield slowly to the pressure; and the tooth makes its way to the surface; or, in common language, is cut. 175. The process of "cutting teeth" is usually not a severe one in the healthy and well-managed infant; but it occasions the death of vast numbers of children who are injudiciously treated; and it is especially fatal to those who have a tendency to disease of the nervous system. The irritation caused by the pressure of the tooth against the gum, is liable to excite, in such cases, convulsive actions of various kinds, on the principles hereafter to be explained (§ 473); and, as the removal of the source of irritation is of the most urgent importance, the lancing of the gums,-doing that in an instant which the pressure of the tooth might not accomplish for days,—is a measure of most obvious utility; however unnecessary it may seem, in ordinary cases, to interfere with the course of nature. But it is of the utmost importance at the same time to bring the nervous system into a less excitable condition; and no measure is commonly more efficacious in this respect, than removal into a fresh and pure atmosphere. 176. At the same time that the development of the tooth is thus taking place, the bone of the jaw is becoming hardened, and closes round its root, forming a complete socket. This partly interrupts the passage of vessels and nerves to the tooth, which, when once fully formed, seems to acquire no further growth, and to possess but little power of repairing injuries occasioned by disease or accident. Hence a tooth which is broken or decayed, is not restored as a bone would be. Still, however, its root or fang is penetrated by a small nerve and artery, which are distributed to the membrane that lines the cavity; and it is to the action of air upon the former, when the cavity is laid open by decay, that the pain of tooth-ache is chiefly due. The remedies which are most effectual in removing this pain, such as kreosote, nitric acid, or a heated wire, are those which destroy the vital power of the nerve. 177. But there are teeth, in many animals, which never cease to grow, and in which the central cavity is always filled 168 TEETH OF RODENTS.- -MOTION OF JAWS. with pulp. Such have no proper root; for additional matter is being continually formed at their base, and thus the whole tooth is pushed upwards. This is the case with the Elephant's tusks; and also with the large teeth that occupy the front of the jaw in Rabbits, Squirrels, Rats, and other gnawing ani Fig. 91.-JAW AND TEETH OF RABBIT. mals (fig. 91). The upper edges of these teeth are being constantly worn away by use : and they are kept up to their proper level by the growth of the tooth from below. But it sometimes happens that one of these teeth is broken off; and the one opposite to it in the other jaw is then thrown into disuse. It continues, however, to grow up from below; but, not being worn down at the top, its length increases greatly, so that it may become a source of great inconvenience to the animal. 178. The teeth are but passive instruments in the act of mastication. They are put in movement by the jaws in which they are fixed; and these are made to act against each other by various muscles. The upper jaw is usually fixed to the head; and has not, therefore, any power of moving independently of it. But the lower jaw is connected with the skull by a regular joint on either side; and is so moved by the muscles attached to it, as to cut, crush, or grind the food, according to the nature of the teeth. 179. There is considerable variety, in different animals, as to the extent of motion which the lower jaw possesses. In the purely Carnivorous quadrupeds, it has merely a hinge-like action, that of opening and shutting; and by the sharpness of the edges of the molar teeth, it is thus rendered a powerful cutting instrument. But in the Herbivorous animals, which have to grind or triturate their food between the roughened surfaces of their molars, such a limited motion would be of no avail; and we accordingly notice, if we watch an ox or a horse whilst masticating its food, that the lower jaw has considerable power of motion from side to side. On the other hand, in the Rodents, or gnawing animals furnished with MOVEMENTS OF LOWER JAW. 169 two large front teeth, the lower jaw has no power of moving from side to side, but is rapidly drawn backwards and forwards; and, as the ridges of the molar teeth are arranged in the opposite direction, they become very powerful filing instruments, by which the toughest vegetable substances are quickly reduced. 180. In the Human jaw, there is a moderate power of motion in all these different directions; and it is furnished with all the muscles by which they are effected in the different animals that perform them; but these are not so large or strong. The most powerful of the muscles of the lower jaw, in all animals, is that by which it is drawn up against the upper, so as to close the mouth. This arises from the side of the skull in the region of the temple, and is hence called the temporal muscle. It covers at its origin a large surface of bone; but its fibres approach one another as they descend, and pass under a bony arch (which may be felt between the cheek and the ear), to attach themselves to a process or projection of the lower jaw (a, fig. 92), about an inch in front of the joint. As the distance from the fulcrum of the point a, at which the power is applied, is thus much less than that of the front of the jaw b, where chiefly the resistance is encountered, the 'power of the muscle is applied at a mechanical disadvantage; and, to overcome a given resistance, the muscle must itself be several times more powerful. Thus the Tiger and Lion, which Fig. 92. HUMAN SKULL. can lift and carry away the bodies of animals weighing several hundred pounds, must possess temporal muscles that shall contract with a force of two thousand, or even more. 181. In Man, as in most of the other Mammalia, there are three kinds of teeth, adapted for different purposes. The first terminate in a thin cutting edge, and are intended simply to divide the food introduced into the mouth; these are termed incisor teeth (fig. 93). Others have more of a conical form, |